We intend to use Drosophila melanogaster to study several aspects of gene function: 1. Gene transcription. In situ hydbridization experiments with polytene chromosomes will be used to analyze populations of cytoplasmic messenger RNA and to compare them with populations of heterogeneous messenger RNA from the same cells. The technique should be sensitive enough to detect changes in RNA populations after specific treatments, such as hormone application, which would be expected to alter genetic activity. 2. Organization of repeated genes. In situ hybridization will be used to investigate the fine structure of the organization of the genes for the five different classes of histone proteins within bands 39D - 39 E on polytene chromosomes. The number of histone gene copies in DNA from adults in stocks bearing duplications and deficiences for polytene region 38D-E will be studied to determine whether there are clusters of histone genes elsewhere in the genome which were not detected by hybridization to polytene chromosomes. Drosophila stocks heterozygous for a deficiency surrounding bands 39 D-E will be used to screen for temperature- sensitive mutations affecting histone production. 3. Polytene chromosome replication. The range of sizes of replicating units in DNA from polytene chromosomes will be determined by autoradiography and compared to the range of sizes of chromomeres as measured cytologically in the same type of chromosomes which are used for the DNA study.